This invention relates to an atomic beam device for frequency standards using an atomic or molecular beam, and in more detail, to a newly designed compact, rigid and inexpensive cesium beam tube which is easy to maintain.
An atomic beam tube which utilizes the atom spectrum of cesium (Cs) for a frequency standard generally includes the basic structure as shown in FIG. 1.
In FIG. 1, the cesium oven 1 which is used as the cesium beam source heats the cesium Cs to a temperature of 80.degree. to 100.degree. C. so that it is vaporized and thereby generates the Cs beam through the collimator.
This Cs beam enters the detector 7 through the first state selection magnet 2 for producing a magnetic field A, high frequency transition part 3, including the microwave cavity 5 placed in the field, for producing a magnetic field C, and the second state selection magnet 6 for producing a magnetic field B. On the other hand, a microwave signal is supplied to an RF input circuit 8. When the frequency of this microwave signal coincides with the transition frequency of the Cs atoms, resonance of the Cs atoms occurs in the cavity 5, resulting in the maximum output of the detector 7. Therefore, a highly stabilized oscillation frequency can be obtained by using closed loop control for the oscillation frequency of the microwave oscillator, for example, such as a crystal controlled oscillator, so that the microwave signal frequency is maintained at the center of the resonance spectrum of the Cs atom. The transition frequency of the Cs atom in the ground state is 9192.631770 MHz.
An example of the conventional cesium beam tube which is configurated by combining the above-mentioned basic elements is shown in the Japanese patent publication No. Toku-Ko-Sho 42-27517 (corresponding U.S. Pat. No. 3,323,008). Moreover, an example of another conventional cesium beam tube is also disclosed in the Japanese patent laid-open application No. Toku-Kai-Sho 51-64895 (corresponding U.S. Pat. No. 3,967,115). The structure of the cesium beam tube in accordance with these prior art references is shown in FIG. 2 in the form of a cross-section.
In FIG. 2, the atomic beam generator 1, A magnetic field unit 2, high frequency transition part 3, B magnetic field unit 6, detector 7, C magnetic field unit 4 and magnetic shield 10 are rigidly and levelly mounted on the mount 9 as shown in FIG. 2. Each element is arranged on the mount 9 so that the atomic beam generated from the atomic beam generator 1 reaches the detector through the specified route G. The mount 9, which is holding each element mentioned above, is provided at the inside of the case 11 which ensures a hermetically sealed structure in combination with the cover 13. The high frequency input circuit 8 is engaged with the case 11 and projected to the outside, and moreover is held to the mount 9 at the section D thereby resulting in a sufficient seal. In addition, at the above-mentioned high frequency signal input port 8, which is coupled to the microwave signal to be supplied from the external circuit, a sealing material 14 is provided, through which said microwave signal effectively passes. In order to hermetically seal each element accommodated in the case 11, the cover 13 is mounted to the case 11 and the area around the joint, namely the part E, is sealed by a means such as welding. In the above-mentioned structure of the atomic beam device, an air exhaust system and vacuum ion pump are connected to an air exhaust port which is provided on case 11 but which is not illustrated in FIG. 2, so that the inside of the atomic beam device is maintained in a vacuum condition. In this case, the ion pump may be built inside of the device or provided as an additional unit outside of the device. After the air in the atomic beam device is exhausted until a vacuum is obtained, the air exhaust system is removed. Then, by sealing said air exhaust port, the atomic beam device can be completed. The part 12 is an airtight electrical connection terminal engaged with the case 11, and is connected to the related portions of the atomic beam device.
However, the existing atomic beam device having the structure mentioned above has always been accompanied by the following problems because the required elements are all housed in the vacuum envelope. The magnets which form the magnetic fields A and B are permanent magnets which are generally manufactured by a method such as casting or sintering. Therefore, such magnets have many fine vacant spaces, which are filled with various kinds of gas. Since it is difficult to sufficiently exhaust gas when sealing the atomic beam device in the vacuum condition, the gas may gradually be released to the inside of the atomic beam device after it is completed. If such gas is released in the atomic beam device, the vacuum is deteriorated, and moreover the Cs source or other portions may be contaminated. Furthermore, the excitation coil used for producing magnetic field C and other internal wiring with an insulation coating also absorbed various kinds of gas, causing the disadvantages mentioned above.
When exhausting the air from the atomic beam device, it is necessary to exhaust the air while the tube is heated to a specified baking temperature in order to exhaust the above-mentioned absorbed gas. This baking temperature is usually 300.degree. C. or higher, which deteriorates or demagnetizes the permanent magnets provided for the magnetic fields A and B. Moreover, the excitation coil for the magnetic field C and the insulation for covering other wires must be heat-resistant. In addition, since the highly permeable magnetic material used as the yoke for said magnetic field C may, for example, be mechanically held together by means of small screws, stress may be generated inside of the yoke material due to differences in the thermal expansion coefficients during said baking, thereby sometimes generating distortion. Such distortion, if it occurs, will result in a non-uniform magnetic field C applied to the atomic beam path of the high frequency transition part. For this reason, it was difficult to exhaust the air from the atomic beam device while maintaining an adequate baking temperature.
Moreover, the air should be exhausted by sustaining the atomic beam device at the baking temperature for a considerable period of time. On the other hand, as described previously, the gas released gradually from the magnets, coil, wiring and other materials even after the air is exhausted deteriorates the vacuum or contaminates the environment, so it was also necessary to sustain the vacuum by additionally providing an ion pump. Said ion pump is positioned outside the atomic beam device or built inside of said device. The former method increases the outside dimensions, while the latter method inevitably results in a complicated device because of the magnet of the ion pump. Furthermore, regardless of how the ion pump is installed, a high voltage power supply is required in order to operate said ion pump, complicating the structure of the device. An extra power supply and facilitates are required for operating the atomic beam device, raising the cost as a whole.
Since each component of the existing atomic beam device is supported on the standard level F of the mount 9, which is spaced far from the atomic beam path G, minute displacements such as twisting or inclination between respective elements influence the atomic beam path G to an undesirably large extent. In order to prevent this disadvantage, it is necessary for mount 9 to be strong enough for maintaining mechanical flatness, so that the material used must have rigidity and sufficient strength. However, particularly if the device is shipped on vehicles, ships or airplanes, a specially designed buffer unit must be added, thereby increasing weight and size, so that the device will survive external forces such as vibration, impact and inertia etc.